The present invention relates to an arrangement for protection against electrical interference in a control computer. The present invention also relates to a method for protecting hardware modules forming part of a control computer against electrical interference and a method of protection against electrical interference emitted by a control computer comprising hardware modules.
Programmable industrial control systems have traditionally been made up from highly specialized hardware and software modules. In recent years, however, there has been increasing interest in also using the so-called personal computer or PC concept for industrial control and automation. This is due to the fact that the personal computer concept is associated with a number of advantages, such as modularity, flexibility, incomparable access to both software tools and hardware modules and high quality, together with well-developed standardization. Personal computers, however, are constructed on the basis of the requirements of an office environment and are therefore in many respects unsuitable for use in more exacting environmental conditions.
Firstly, contacts and other mechanical parts are often designed for an office environment and are not of sufficient mechanical and electrical quality for an industrial environment. Secondly, the mechanical and electrical design of hardware modules of personal computers is such that modules cannot be replaced during operation. This facility is extremely desirable in the case of industrial control in order to avoid having to shut down an entire production line in the event of a failure in a module. Furthermore the electrical design of hardware modules is often far too deficient for industrial use. The great majority of modules on the market lack basic electrical protection, which is always required in an industrial context in order to prevent the destructive or inconvenient effect of incident electromagnetic interference, whether line-conducted or transmitted through capacitance/inductance. Even if specially manufactured hardware modules are used with complete protection, the problem remains that interference is still given off in the vicinity of modules that do not have corresponding protection, which in turn leads to a risk of capacitive/inductive transmission of the interference. Furthermore the main components according to the PC concept are basically not designed to withstand the levels of interference that an industrial control system i s expected to withstand. By main components we mean here the CPU and the so-called PC bus for electrical communication between the CPU and the constituent hardware modules. The said hardware modules must therefore be protected from the risk of line-conducted or capacitive/inductive interference.
An hermetically sealed construction for use in difficult environmental conditions, especially in military environments, is described in U.S. Pat. No. 5,570,270. The construction comprises a personal computer and a casing containing printed circuit cards and is intended to protect the printed circuit cards from electromagnetic interference (EMI) and mechanical influences in the form of high accelerations.
The present invention is intended to produce an improved arrangement and methods compared to the prior art in a control computer for use, for example, in an industrial or medical environment, which provide a good compromise between good protection against interference on the one band and flexibility and interchangeability on the other.
This is made possible by an arrangement of the type stated in the introductory part, which is characterised in that the control computer comprises hardware modules designed to personal computer standards, in that at least one of the modules is connected by way of first leads to means adapted to prevent at least the line-conducted interference, which is harmful to the modules, from reaching the latter, in that the interference-preventing means are connected by way of second leads to the external unit and that the arrangement comprises means adapted to shield the modules and the connection between the modules and the interference-prevent means in order to prevent capacitive/inductive transmission of the interference, the said means of shielding comprising an outer casing, essentially enclosing at least the modules and the connection between the modules and the interference-preventing means and shielding them against electromagnetic radiation, together with an inner casing essentially enclosing at least one of the modules and shielding it against electromagnetic radiation.
This has also been made possible by means of methods of the type stated in the introductory part, which are characterised in that at least the signals from the external unit to each module are made to pass through interference-preventing means, which prevent the signal interference that is harmful to the modules from reaching the latter, that the control computer is essentially enclosed by an outer casing, shielding it against interference in the form of electromagnetic radiation, and that the hardware modules are essentially enclosed by an inner casing shielding them against the said electromagnetic interference, in which the parts of the control computer that are located inside the inner casing are designed in accordance with a personal computer standard and the parts of the control computer that are located outside the inner casing are designed in accordance with a standard for use in some industrial environment, for example DIN. The casings are adapted both to protect the modules from external electromagnetic radiation and to protect the surroundings from electromagnetic radiation that is emitted by the modules. In addition the signals from each module to the external unit can also be made to pass through the interference-preventing means, which prevent signal interference in excess of a certain energy level from passing.
The present invention has a number of improvements on the prior art. Among other things, shielding against electromagnetic radiation is produced in two stages, which function even in environments very susceptible to interference, for example hospital environments, where interference from and by equipment located in the vicinity is entirely unacceptable. In addition high energies are prevented from reaching the unshielded parts of the control computer by way of the leads. Furthermore the method and the arrangement according to the invention are, for example, suitable for use in the control and automation of various processes in industry. Valuable advantages in this and other contexts include the fact that the arrangement according to the invention can be manufactured at relatively low cost, due among other things to the use of standard components and the simplicity of the arrangement, and to the fact that it is possible to replace components during operation.